Device-to-device (D2D) communications is one means for improving the performance of LTE (Long Term Evolution) and other cellular networks. In D2D communications, terminals (referred to as user equipments or UEs in LTE) communicate with one another directly rather than being linked through the base station (referred to as an evolved node B or eNB in LTE). D2D communication between two or more D2D devices is typically very local, due to the short distance between D2D devices and uses very lower transmit power. D2D communications is also a powerful way to increase spatial reuse in cellular systems for higher throughput.
One approach to D2D communications as an underlay to an LTE network infrastructure is an out-of-band solution, in which the D2D traffic is unloaded to an unlicensed band (e.g., Wi-Fi as defined by the IEEE 802.11 standards) on the application layer. Another approach is an in-band solution, in which the D2D transmissions take place in the same licensed band used by the LTE network.